1. Field of the Invention
The invention relates to a device for reversible optical data storage, using amorphous polymers in a tough and resilient state and in a vitreous solidified state.
2. Discussion of the Background
In addition to a solid crystalline state and a liquid melt state, polymer systems in a limited range of temperatures can have a highly viscous, tough and resilient state and at lower temperatures the solid, vitreous solidified state. By means of suitable chemical composition of the polymer molecules or by means of suitable adjustment of the chain length of the molecules, the tendency to form these states can be increased. Homopolymers, copolymers, branched polymers and cross-linked polymers have an equivalent tendency to form said states. (See H.G. Elias, Makromolekule, Huthig and Wepf Verlag, Basel; B. Wunderlich, Macromolecular Physics, Academic Press, New York, 1973).
The existence of the vitreous solidified state can be characterized, for example, by means of calorimetric or dilatometric analysis methods. At the transition of a glassy state into the tough and resilient state, the specific heat and the coefficient of thermal expansion increase sharply. The temperature range for the existence of the tough and elastic state can be determined by means of rheological methods or dynamic-mechanical analysis methods (See D. W. van Krevelen, Properties of Polymers, Elsevier Scientific Publishing Company, Amsterdam, 1976; H. G. Elias, loc., cit; B. Wunderlich loc. cit).
With respect to their structure, the tough and resilient state and the vitreous solidified state are generally characterized by the fact that there is no periodic, crystalline arrangement of the molecular groups and that the molecular groups have a statistical orientation. This is true as long as no liquid crystalline polymers are considered and as long as during cooling, no external electric, magnetic or mechanical fields act on the system.
In the simplest case, the orientational order of the tough and resilient or vitreous solidified state can be characterized by means of the orientation order parameter f.sub..theta.. (See P. G. de Gennes "The Physics of Liquid Crystals"0 Clarendon Press, Oxford, 1974; W. H. de Jeu, Physical Properties of Liquid Crystalline Materials, Gordon and Breach Science Publishers, 1980). In this case the orientation parameter is defined as follows: EQU f.sub..theta. =1/2&lt;3 cos.sup.2 .theta.-1&gt;
wherein .theta. is the angle between the longitudinal axis of the molecular group (segment of the chain) and a preferred direction. The value of this order parameter is 1 for a complete, perfectly parallel arrangement of all molecular groups and zero for a statistical orientation distribution. This value is generally observed for the tough and resilient and the glass-like solidified state of amorphous polymers and is, in particular, homogeneously observed for the entire sample. (See F. Bueche, Physical Properties of Polymers, Interscience, New York, 1962).
The density is also homogeneous for the entire sample and, in particular, not only for the vitreous solidified state but also for the tough and resilient amorphous state. It follows directly that optical properties, such as the refractive index and the double refraction, are homogeneous for the samples, whereby in the case of amorphous polymers the amount of double refraction is uniformly zero. Consequently polymers are transparent in both of these states; and under crossed polarization, both of the states appear black. The wide spread use of amorphous polymers in industrial products such as films, photoconductors, special glasses or industrial glazing materials is based on these special optical properties.
Recently a number of industrial uses of amorphous polymers in the tough and resilient and vitreous solidified states have become known in the field of optical storage. Thus, recently a number of photopolymers have been proposed for recording phase holograms. (See P. Hariharan, Optical Holography, Cambridge, University Press, 1984; Sh. Reich, Angew. Chemie, Vol. 89, 467, 1977; H. M. Smith, Holographic Recording Materials, Springer Verlag, Berlin, 1977; W. Driemeier, M. Kopietz, M. D. Lechner, Colloid Polym. Sci., 264, 1024, 1986). In this case, they are blends of monomers or oligomers with light sensitive catalysts. The active mechanism for information storage is based on the fact that the refractive index changes with the molecular weight. A local non-homogeneous light distribution produces a non-homogeneous polymerization and induces thereby, a non-homogeneous distribution of the refractive index. In an analogous manner the refractive index can be spatially varied by means of photochemical cross-linking. (See Sh. Reich, loc. cit). An important drawback of this method is that it involves an irreversible information storage and that diffusion processes occur due to the presence of molecules having varying chain lengths, which result in a negative impact on the optically stored information.
Furthermore, in the literature, the possibility of writing optically accessible information (by means of embossing amorphous polymers) into the surface of amorphous polymers is described. This process is used in the case of compact disc ROM's or audio-compact discs. It has the drawback that data cannot be erased or recorded again. (See J. Hennig, Kunststoffe, Vol. 75, 7, 1985, Philips Technical Review, Vol. 40, 6, 1982).
A process is also reported that starts from dye-doped amorphous polymer films, which when applied to suitable systems permit information to be optically recorded in the form of holes or bubbles. Even this process does not permit data to be erased or rewritten. (See M.Law, D. Johnsen, J. Appl. Phys., Vol. 54, 9, 1983).
The photochemical hole burning method makes use of the selective bleachability of the absorption lines of dye molecules in amorphous polymer matrices. The starting point is the non-homogeneous broadening of spectral lines through "side resonances", in which at low temperatures, frequency holes with high information density can be reversibly recorded. This process has, among others, the drawback that it requires low temperatures when writing, storing or reading (See A. Gutierrez, J. Friedrich, D. Haarer, H. Wolfrom, IBM Journal of Research and Development, Vol. 26, 2, 1982).
Other storage methods, based on thermoplastic polymers, are based on the deformability of the polymer surface under the influence of electrostatic forces. The resulting surface relief then serves as a phase modulator for a transmitted or reflected illuminating wave. The required surface load picture is generated by means of a sandwich constructed of a thermoplastic, photoconductor, and conductor, on which the two dimensional optical information is exposed. In this case, the optical information can be erased and rewritten again. The drawback of this process is the quite complex construction of the sandwich and the fact that the entire write--erase procedure requires several complicated process steps. (See H. M. Smith, Holographic Reading Materials).
Now, as before, there is a great interest in optical storage media, which have not only high recording densities but also the possibility of reversible storage of information. The above described solutions to the problem of optical data storage in amorphous polymers are relatively narrowly defined engineering solutions. Thus in many cases the data cannot be reversibly stored. In other cases the construction of the storage medium is complex. The processes required for storing are time-consuming or from an engineering point of view, the temperature limitations for the storage medium result in processes which are not very practical.